I've got a gut-related condition and saw this claim on the BBC
[The stomach's] complex work is under the control of what's sometimes called "the little brain", a network of neurons that line your stomach and your gut. Surprisingly, there are over 100 million of these cells in your gut, as many as there are in the head of a cat.
The little brain does not do a lot of complex thinking but it does get on with the essential daily grind involved in digesting food - lots of mixing, contracting and absorbing, to help break down our food and begin extracting the nutrients and vitamins we need.
It seems profoundly unlikely to me that the human digestive system requires as many neurons as an entire small animal. Are there really so many neurons in the human gut?